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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="overflow.html" title="cpp/io/basic filebuf/overflow"><span class="t-lines"><span>basic_filebuf::overflow</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="setbuf.html" title="cpp/io/basic filebuf/setbuf"><span class="t-lines"><span>basic_filebuf::setbuf</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>basic_filebuf::seekoff</span></span></strong></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="seekpos.html" title="cpp/io/basic filebuf/seekpos"><span class="t-lines"><span>basic_filebuf::seekpos</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="sync.html" title="cpp/io/basic filebuf/sync"><span class="t-lines"><span>basic_filebuf::sync</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="imbue.html" title="cpp/io/basic filebuf/imbue"><span class="t-lines"><span>basic_filebuf::imbue</span></span></a></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5">非成员函数</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="swap2.html" title="cpp/io/basic filebuf/swap2"><span class="t-lines"><span>swap<span class="t-dsc-small">(std::basic_filebuf)</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>

<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">protected</span><span class="sy4">:</span><br>
<p><span class="kw1">virtual</span> pos_type seekoff<span class="br0">(</span> off_type off,<br>
                          <a href="../ios_base/seekdir.html"><span class="kw1702">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">seekdir</span></span></a> dir,<br>
</p>
                          <a href="../ios_base/openmode.html"><span class="kw1671">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">openmode</span></span></a> which <span class="sy1">=</span> <a href="../ios_base/openmode.html"><span class="kw1674">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">in</span></span></a> <span class="sy3">|</span> <a href="../ios_base/openmode.html"><span class="kw1675">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">out</span></span></a> <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>若可能，则重寻位文件指针到距文件起始、结尾或当前位置准确 <code>off</code> 个字符的位置（取决于 <code>dir</code> 的值）。
</p><p>若关联文件未打开（ <span class="t-c"><span class="mw-geshi cpp source-cpp">is_open<span class="br0">(</span><span class="br0">)</span><span class="sy1">==</span><span class="kw2">false</span></span></span> ），则立即失败。
</p><p>若多字节字符编码依赖状态（ <a href="../../locale/codecvt/encoding.html" title="cpp/locale/codecvt/encoding"><tt>codecvt::encoding()</tt></a> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">1</span></span></span> ）或为变长（ <code>codecvt::encoding()</code> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ），而偏移 <code>off</code> 非 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ，则立即失败：此函数无法确定对应 <code>off</code> 个字符的字节数。
</p><p>若 <code>dir</code> 不是 <span class="t-lc"><a href="../ios_base/seekdir.html" title="cpp/io/ios base/seekdir">std::basic_ios::cur</a></span> 或偏移 <code>off</code> 非 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ，且此 filebuf 对象上最近做的操作是输出（即放置缓冲区非空，或最近调用的函数是 <span class="t-lc"><a href="../basic_streambuf/overflow.html" title="cpp/io/basic streambuf/overflow">overflow()</a></span> ），则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">codecvt</span><span class="sy4">::</span><span class="me2">unshift</span></span></span> 确定需要的反迁移序列，并通过调用 <span class="t-lc"><a href="../basic_streambuf/overflow.html" title="cpp/io/basic streambuf/overflow">overflow()</a></span> 写入该序列到文件。
</p><p>然后转换参数 <code>dir</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> 类型值 <code>whence</code> 如下：
</p>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> <code>dir</code> 的值
</td>
<td> <code>whence</code> 的值
</td></tr>


<tr class="t-dsc">
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_ios</span><span class="sy4">::</span><span class="me2">beg</span></span></span>
</td>
<td>  <span class="t-lc"><a href="../c.html" title="cpp/io/c">SEEK_SET</a></span>
</td></tr>

<tr class="t-dsc">
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_ios</span><span class="sy4">::</span><span class="me2">end</span></span></span>
</td>
<td>  <span class="t-lc"><a href="../c.html" title="cpp/io/c">SEEK_END</a></span>
</td></tr>

<tr class="t-dsc">
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_ios</span><span class="sy4">::</span><span class="me2">cur</span></span></span>
</td>
<td>  <span class="t-lc"><a href="../c.html" title="cpp/io/c">SEEK_CUR</a></span>
</td></tr>
</table>
<p>然后，若字符编码为定宽（ <code>codecvt::encoding()</code> 返回某个正值 <code>width</code> ），则如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../c/fseek.html"><span class="kw1867">std::<span class="me2">fseek</span></span></a><span class="br0">(</span>file, width<span class="sy2">*</span>off, whence<span class="br0">)</span></span></span> 移动文件指针。
</p><p>否则，如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../c/fseek.html"><span class="kw1867">std::<span class="me2">fseek</span></span></a><span class="br0">(</span>file, <span class="nu0">0</span>, whence<span class="br0">)</span></span></span> 移动文件指针。
</p><p>基类函数签名所要求的 <code>openmode</code> 参数通常被忽略，因为 <code>std::basic_filebuf</code> 只维护一个文件位置。
</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  off
</td>
<td> -
</td>
<td> 要设置位置指示器到的相对位置。
</td></tr>
<tr class="t-par">
<td>  dir
</td>
<td> -
</td>
<td> 定义要应用相对偏移到的基位置。它能为下列常量之一：
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td>  常量
</td>
<td>  解释
</td></tr>


<tr class="t-dsc">
<td>  <a href="../ios_base/seekdir.html" title="cpp/io/ios base/seekdir"><tt>beg</tt></a>
</td>
<td>  流的开始
</td></tr>

<tr class="t-dsc">
<td>  <a href="../ios_base/seekdir.html" title="cpp/io/ios base/seekdir"><tt>end</tt></a>
</td>
<td>  流的结尾
</td></tr>

<tr class="t-dsc">
<td>  <a href="../ios_base/seekdir.html" title="cpp/io/ios base/seekdir"><tt>cur</tt></a>
</td>
<td>  流位置指示器的当前位置
</td></tr>
</table>
</td></tr>
<tr class="t-par">
<td>  which
</td>
<td> -
</td>
<td> 定义要影响输入和/或输出序列的哪一个。它能为下列常量之一或其组合：
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> 常量
</td>
<td> 解释
</td></tr>


<tr class="t-dsc">
<td>  <a href="../ios_base/openmode.html" title="cpp/io/ios base/openmode"><tt>in</tt></a>
</td>
<td> 影响输入序列
</td></tr>

<tr class="t-dsc">
<td>  <a href="../ios_base/openmode.html" title="cpp/io/ios base/openmode"><tt>out</tt></a>
</td>
<td> 影响输出序列
</td></tr>
</table>
</td></tr></table>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
<p>新构造的 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos_type</span></span> 类型对象，存储结果文件位置，或在失败时为 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos_type<span class="br0">(</span>off_type<span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span></span></span> 。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<p><code>seekoff()</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_streambuf</span><span class="sy4">::</span><span class="me2">pubseekoff</span></span></span> 所调用，它又为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_istream</span><span class="sy4">::</span><span class="me2">seekg</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_ostream</span><span class="sy4">::</span><span class="me2">seekp</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_istream</span><span class="sy4">::</span><span class="me2">tellg</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_ostream</span><span class="sy4">::</span><span class="me2">tellp</span></span></span> 所调用。
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;fstream&gt;</span>
<span class="co2">#include &lt;locale&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// 准备 10 字节文件，保有 4 个 UTF8 中的字符</span>
    <a href="../basic_ofstream.html"><span class="kw1733">std::<span class="me2">ofstream</span></span></a><span class="br0">(</span><span class="st0">"text.txt"</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> u8<span class="st0">"z<span class="es3">\u00df</span><span class="es3">\u6c34</span><span class="es4">\U0001d10b</span>"</span><span class="sy4">;</span> <span class="co1">// 或 u8"zß水𝄋"</span>
                                           <span class="co1">// 或 "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";</span>
 
    <span class="co1">// 用非转换编码打开</span>
    <a href="../basic_ifstream.html"><span class="kw1732">std::<span class="me2">ifstream</span></span></a> f1<span class="br0">(</span><span class="st0">"text.txt"</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f1's locale's encoding() returns "</span>
              <span class="sy1">&lt;&lt;</span> <a href="../../locale/use_facet.html"><span class="kw1897">std::<span class="me2">use_facet</span></span></a><span class="sy1">&lt;</span><a href="../../locale/codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="kw4">char</span>, <a href="../../string/multibyte/mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">&gt;&gt;</span><span class="br0">(</span>f1.<span class="me1">getloc</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>.<span class="me1">encoding</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"pubseekoff(3, beg) returns "</span> <span class="sy1">&lt;&lt;</span> f1.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>pubseekoff<span class="br0">(</span><span class="nu0">3</span>, <a href="../ios_base/seekdir.html"><span class="kw1703">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">beg</span></span></a><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"pubseekoff(0, end) returns "</span> <span class="sy1">&lt;&lt;</span> f1.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>pubseekoff<span class="br0">(</span><span class="nu0">0</span>, <a href="../ios_base/seekdir.html"><span class="kw1705">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">end</span></span></a><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;;</span>
 
    <span class="co1">// 用 UTF-8 打开</span>
    <a href="../basic_ifstream.html"><span class="kw1746">std::<span class="me2">wifstream</span></span></a> f2<span class="br0">(</span><span class="st0">"text.txt"</span><span class="br0">)</span><span class="sy4">;</span>
    f2.<span class="me1">imbue</span><span class="br0">(</span><a href="../../locale/locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="br0">(</span><span class="st0">"en_US.UTF-8"</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f2's locale's encoding() returns "</span>
              <span class="sy1">&lt;&lt;</span> <a href="../../locale/use_facet.html"><span class="kw1897">std::<span class="me2">use_facet</span></span></a><span class="sy1">&lt;</span><a href="../../locale/codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1">&lt;</span><span class="kw4">wchar_t</span>, <span class="kw4">char</span>, <a href="../../string/multibyte/mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">&gt;&gt;</span><span class="br0">(</span>f2.<span class="me1">getloc</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>.<span class="me1">encoding</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"pubseekoff(3, beg) returns "</span> <span class="sy1">&lt;&lt;</span> f2.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>pubseekoff<span class="br0">(</span><span class="nu0">3</span>, <a href="../ios_base/seekdir.html"><span class="kw1703">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">beg</span></span></a><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"pubseekoff(0, end) returns "</span> <span class="sy1">&lt;&lt;</span> f2.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>pubseekoff<span class="br0">(</span><span class="nu0">0</span>, <a href="../ios_base/seekdir.html"><span class="kw1705">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">end</span></span></a><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">f1's locale's encoding() returns 1
pubseekoff(3, beg) returns 3
pubseekoff(0, end) returns 10
f2's locale's encoding() returns 0
pubseekoff(3, beg) returns -1
pubseekoff(0, end) returns 10</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../basic_streambuf/pubseekoff.html" title="cpp/io/basic streambuf/pubseekoff"> <span class="t-lines"><span>pubseekoff</span></span></a></div></div>
</td>
<td>  调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">seekoff<span class="br0">(</span><span class="br0">)</span></span></span> <br> <span class="t-mark">(<code>std::basic_streambuf&lt;CharT,Traits&gt;</code> 的公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="seekpos.html" title="cpp/io/basic filebuf/seekpos"> <span class="t-lines"><span>seekpos</span></span></a></div><div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div></div>
</td>
<td>  用绝对寻址重寻位文件位置 <br> <span class="t-mark">(虚受保护成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../c/fseek.html" title="cpp/io/c/fseek"> <span class="t-lines"><span>fseek</span></span></a></div></div>
</td>
<td>  移动文件位置指示器到文件中的指定位置 <br> <span class="t-mark">(函数)</span> </td></tr>
</table>

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